A phase-shifted full-bridge DC/DC converter, in its basic form may include a primary side containing a set of switches used to control the application of an input voltage to the primary coil of a transformer, and a secondary side containing a set of output diode rectifiers that serve to produce a voltage with constant polarity. The converter may be in an active state, during which a differential voltage is applied across the primary coil of the transformer, or in a passive state, during which the same voltage is applied to both terminals of the transformer's primary coil. Thus, electrical energy flows in the converter during the active state.
Switching losses occur when the semiconductor switches transition from on to off or vice versa. In order to achieve high efficiency in a phase-shifted full-bridge DC/DC converter, low switching losses are maintained over a wide range of operating conditions. This is typically achieved by adding a “resonant inductor” to the primary side of the converter and adding “dead time” delay between the two switches in each half-bridge to allow time for the output voltage of the half-bridge to commutate prior to activating the opposing switch. However, the inductive source impedance causes voltage overshoot and ringing during the decaying portion of the reverse current that occurs in the output rectifier diodes following a state transition in the converter. This voltage overshoot and ringing can generate excessive dynamic losses, unacceptable EMI, and increased voltage stress on the diodes at the secondary-side of the converter. Failure of the rectifier diodes can occur due to the increased voltage stress. A typical solution to this problem involves clamping the junction between the transformer and the resonant inductor to the supply rails with two diodes, known as clamp diodes.